My buddy just got his 95 V6 back up and running after 2 yrs of it sitting due to a head gasket that had blown. Now at idle, it has a surge to it. RPM go up, down to where it almost dies, then back up, back down and almost dies. Could this be bad gas? Or possibly a vacuum leak somewhere. I told him that i heard a hiss from a vac leak but he doesn't think that it is. Any thoughts??

Thanks!
Jason